The present invention relates to a mass spectrometry and equipment for carrying out the same, and in particular to the technology of element isotope identification in a mass spectrometer using a plasma ion source.
Conventionally, when carrying out isotope identification of an ion using a mass spectrometer the method of carrying out analysis using a high-resolution mass spectrometer (the milli-mass method) was being used widely. In this method, the mass numbers in which the target ion is present are scanned using a high-resolution mass spectrometer, the minute mass difference (several milli-amu to several tens of milli-amu) resulting out of the mass loss effect is measured, thereby carrying out the identification of the isotope of the material present for that mass number. In this method, since it is easy to detect the amount present of each elemental ion individually, it is easily possible to judge the amount of target ions present. An example of this method is disclosed in Japanese Patent Publication No. Hei 11-260310 and other publications.
In recent years, although the resolution is low, since the equipment itself is small-sized and also simple and has the feature of being relatively inexpensive, the quadruple-electrode type mass spectrometer is started to being used very widely. Recently, even with such relatively simple and low-resolution quadruple-electrode type mass spectrometers, there is an increasing need for carrying out more detailed ion analysis.
However, in such mass spectrometers, since the resolution is low, being about several tens of milli-amu or more at different mass numbers, it is very difficult to detect very small mass differences of each element. In this context, generally, the method of isotope identification using the ratio of presence of isotopes of the element is being used as a different approach. In this method, for the target ion, the mass numbers in which the element isotopes containing that ion are scanned over a range of several to several tens of amu, and the composition of the element is analyzed by comparing with the presence ratio of the isotopes of the element. An example of this method has been disclosed in Japanese Patent Publication No. Hei 8-17391.
These examples are methods of composition analysis mainly with molecular ions as the target. There is also a demand for measuring only a specific elemental ion with a low-resolution mass spectrometer. As a particular example of this, is the city water supply potable water quality standards announced by the Ministry of Health and Welfare, and to judge these standards, an analyzing equipment is required that is capable of detecting the amount of metallic elements contained from a concentration level of several tens of parts per billion.
As an analyzing equipment for use in such fields, the mass spectrometer based on the inductive coupling plasma method (ICP-MS) has received attention because it has a high sensitivity although its mass number resolution is relatively low.
Even in such a field of analyzing elemental ions, since all the elements present in the specific mass number can be detected in a superimposed manner in a low resolution mass spectrometer, when measuring an unknown sample, it becomes necessary to eliminate unnecessary interference ions and extract only the target elemental ions.
As a method of eliminating such interference ions, conventionally, there was the method in which the user specifies the elements that are considered to be interference ions, such interference ions are measured along with the target elements, and from the results of such measurements, the amount of interference is calculated and removed.
In particular, regarding the elimination of interference due to molecular ions, as has been described in Japanese Patent Publication No. Hei 10-274640, a method has been disclosed regarding molecular ions, in which the correction amount is calculated using the ratio of isotope presence of specific elements. In this method, the procedure has been described of removing from the mass amount spectrum the mass peaks due to carbon isotopes using the presence ratio of the carbon mass numbers 12 and 13. However, there is no disclosure as to how to eliminate the interference due to elements other than carbon.
In the case of the method disclosed in Japanese Patent Publication No. Hei 10-274640, the user alone has to give considerations to the interference, and hence there were failures in measurement caused by insufficient considerations given by the user. In the field of analysis of small amounts of samples in which the measurements have to be made using limited samples, such mistakes in measurement can be fatal, and hence a method of eliminating the interference automatically, easily, and definitely was necessary.
When carrying out element analysis using a relatively low resolution mass spectrometer, it is common to measure the amount of ions detected in the isotope mass numbers of the target element, and to quantify the concentration value from that detected amount. However, since there are overlapping mass numbers among the isotopes of different elements, there are times when there other overlapping element isotopes or molecules at the target element isotope""s mass number to be measured, and in such a case, the measurement result will contain interference from other elemental ions or molecular ions. In order to remove this interference, it was necessary that the user predict the element or the molecule whose mass number overlaps with that of the target, and in order to calculate the amount of that element or molecule, first measuring the isotope mass numbers of the interfering ions, and then calculating and eliminating the amount of interference for the target ion using the isotope presence ratio unique to that element.
In this method, if the user makes a mistake in considering the effect of interference or does not consider it at all, since the amount of interference will remain as such as a measurement error, a method was necessary for definitely and easily eliminate the amount of interference.
Further, in a mass spectrometer used for element analysis, since only the target mass numbers are scanned from the point of view of increasing the throughput, etc., the method of use of scanning a wide range of mass numbers only for the purpose of confirming the interference from other elements is not realistic, and a means was necessary for appropriately selecting the mass numbers necessary for eliminating interference.
The purpose of the present invention is to provide a mass spectrometry and a mass spectrometer using that method, which permit the measurement of the target ion amount easily and in a short time, even when other elemental ions cause interference to the target elemental ion at the time of measuring the amounts of isotope ions of a specific element.
In order to solve the above problems, the feature of the present invention is a mass spectrometry containing a step of setting the element to be measured as the first measurement target, a step of searching, using a previously registered element isotope information, for the element interfering with the first measurement target that has been set above, when an interfering element is found to be present from this search, a step of selecting the second measurement target from among the isotopes of that interfering element, a step of carrying out the measurements of said first measurement target and second measurement target, and a step of calculating the amount of ions of said first measurement target using the ion amount measurement result of said second measurement target.
Further, an additional feature of the present invention is that, in a mass spectrometer having an ion source that ionizes the sample, a mass analyzer section that carries out mass spectrometry of the sample ionized by the ion source, a detector section that detects the sample ions after mass spectrometry, and a data processing provided with a display section and an input section for carrying out the settings of different sections and display of the detected results, said data processing section has a storage section in which the information of element isotopes and isotope presence ratio is stored, and also said display section has an element selection area for specifying the element to be measured and an isotope information window for displaying the isotopes of the element selected in said element selection area, and the measurement target to be determined by selecting any isotope from the isotopes displayed in said isotope information window.